We studied the population and coherence dynamics of a biexciton in a planar CuCl microcavity. We developed a time-resolved two-photon polarization spectroscopy technique, by which we extracted the population dynamics of the biexciton even when the biexciton was spectrally overlapped with the lower cavity polariton. The observed lifetime of the cavity biexciton in the weak-coupling regime was much shorter than that of a bare CuCl thin film without a cavity, which directly demonstrates cavity enhancement of the radiative decay rate of the biexciton. The cavity enhancement originates from the hybridization of the unbound two-cavity-polariton state to the biexciton wave function by the biexciton-cavity coupling. By comparing the population and coherence decay curves of the biexciton, we found that the coherence of the cavity biexciton was limited by the radiative population decay.